A frame structure of a TDD mode of a Long Term Evolution (LTE) system (also called as a frame structure type 2) is as shown in FIG. 1. In this kind of frame structure, a 10 ms (307200 Ts, 1 ms=30720 Ts)-radio frame is divided into two half-frames, each half-frame is 5 ms (153600 Ts) long. Each half-frame contains 5 subframes with a length of 1 ms respectively. The function of each subframe is as shown in Table 1, wherein, D represents a downlink subframe used for transmitting downlink signals. U represents an uplink subframe used for transmitting uplink signals. An uplink subframe or a downlink subframe is then divided into 2 0.5 ms-time slots. S represents a special subframe, which contains 3 special time slots, namely a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS). In an actual system, a terminal will be informed of uplink/downlink configuration indexes through broadcast message.
TABLE 1Uplink/downlink configurationConfig-Switch-pointSubframe numberurationperiodicity012345678905msDSUUUDSUUU15msDSUUDDSUUD25msDSUDDDSUDD310msDSUUUDDDDD410msDSUUDDDDDD510msDSUDDDDDDD65msDSUUUDSUUD
The resource allocation in the LTE system takes a Resource Block (RB) as a unit, an RB occupies 12 Resource Elements (REs) in a frequency domain, an RE occupies one Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbol in a time domain, an RB occupies one time slot in the time domain, that is, an RB occupies 7 SC-FDMA symbols under the condition of normal Cyclic Prefix (CP) and occupies 6 SC-FDMA symbols under the condition of extended CP. A relation between the resource block and the resource unit is as shown in FIG. 2.
In the LTE system, uplink/downlink data transmission can use the Hybrid Automatic Repeat Request (HARQ) technology.
The uplink data transmission uses the synchronous adaptive HARQ technology or the synchronous non-adaptive HARQ technology, and a main process includes the following steps.
In step (1), a base station informs a terminal of information such as time-frequency resources and a modulation coding scheme and so on used in the data transmission through uplink scheduling information (Uplink Grant) in a downlink control signaling; and the uplink scheduling information is sent through an uplink scheduling grant signaling.
In step (2), the terminal uses a Physical Uplink Shared Channel (PUSCH) to send uplink data according to the information.
In step (3), the base station decodes the uplink data after receiving the uplink data, and informs the terminal of a decoding result. If receiving the uplink data correctly, the base station sends an Acknowledgement (ACK), and if receiving the uplink data incorrectly, the base station sends a Negative Acknowledgement (NACK). With regard to the adaptive HARQ technology, when the data are received incorrectly, the base station also can send UL GRANT to change information such as a modulation coding scheme and time-frequency resources of a retransmission packet besides sending the NACK.
In step (4), the terminal uses the PUSCH to resend uplink data (i.e., the retransmission packet) after receiving the NACK.
In step (5), the base station combines the uplink data with respect to the same data packet sent at each time and decodes the combined uplink data. Then, the operations of step (3) and step (4) are repeated until the number of times of retransmission reaches a maximum value regulated by the system.
A data packet occupies one HARQ process. In order to enhance the usage efficiency of system resources, during the process of transmitting or retransmitting a data packet, the base station also can use other processes to transmit a new data packet. For example, a TDD uplink/downlink configuration 1 supports 4 processes at most (a timing relationship is as shown in FIG. 3), when a data packet in a process 1 is transmitted (it is assumed that a data packet of the process 1 is transmitted on a subframe 2 of a radio frame n, if this data packet is transmitted incorrectly, a subframe 2 of a radio frame n+1 is a retransmission packet of this data packet; and if this data packet is transmitted correctly, the subframe 2 of the radio frame n+1 is another new data packet of the process 1), the base station can use a process 2, process 3 and process 4 to transmit new data packets.
Since the synchronous HARQ technology is used in uplink, there exists a fixed timing relationship between the Uplink Grant, PUSCH and ACK/NACK. With regard to the TDD mode, this timing relationship is related to uplink/downlink configurations. When the Uplink Grant or ACK/NACK are sent on a subframe n and the PUSCH is sent on a subframe n+k, a value of k is as shown in Table 2. In addition, with regard to an uplink/downlink configuration 0, for the ACK/NACK sent on a subframe 1 and a subframe 6, PUSCH is sent on a subframe n+7, and for the ACK/NACK sent on a subframe 0 and a subframe 5, PUSCH may be sent on a subframe n+4 or the subframe n+7. For the Uplink Grant sent on the subframe 0, subframe 1, subframe 5 and subframe 6, PUSCH may be sent on the subframe n+k and/or the subframe n+7.
Note: when a value of the n+k does not belong to the subframe scope (0˜9) of the current radio frame, the n+k indicates a corresponding subframe within the next radio frame. For example, when n=6 and k=6, the n+k indicates a subframe 2 of the next radio frame.
TABLE 2TDD UL/DLsubframe number nConfiguration01234567890464616464244344444454677775
The ACK/NACK sent on a subframe i corresponds to the PUSCH sent on a subframe i-k, a value of k is as shown in Table 3. In addition, with regard to the uplink/downlink configuration 0, the ACK/NACK sent on the subframe 0 and the subframe 5 also may correspond to the PUSCH sent on a subframe i-6.
TABLE 3TDD UL/DLsubframe number iConfiguration01234567890747414646266366646656664746
In order to implement an Enhanced Inter-Cell Interference Coordination (EICIC) better, the Release (R) 10 of LTE-Advanced supports the cross-carrier scheduling, that is, control information and data are not sent on the same carrier. As shown in FIG. 4, scheduling information and ACK/NACK of the PUSCH of a Component Carrier (CC) 1 are sent on a CC 2.
With regard to the TDD mode, the R10 of LTE-Advanced only supports the carrier aggregation within the same band as shown in FIG. 4. At this point, different CCs are required to use the same uplink/downlink configuration, thus different CCs have the identical HARQ timing relationship, and no problem will occur in the cross-carrier scheduling.
In order to utilize the resources more flexibly, the R11 of LTE-Advanced will support the carrier aggregation of different bands, and carriers located in different bands can be configured with different uplink/downlink proportions. Since different uplink/downlink proportions have different HARQ timing relationships, when different uplink/downlink configurations are used on different carriers, how to make maximal use of the existing HARQ timing relationships to implement the cross-carrier scheduling and maintain a lower complexity of devices in the meantime is a problem required to be solved urgently.